Chronic or persistent pain is different from acute pain and can be harder to recognize, because its onset is slow, its intensity is likely not constant, it is not necessarily associated with an obvious pathologic condition, and it usually does not serve any vital protective function. It is also more likely to lead to distress and maladaptive behavior.
Animals in chronic pain can be divided into three broad categories: those with a known pathologic condition (e.g., arthritis, cancer, or injury), those in which an organic cause of the pain can be inferred from the results of the clinical examination and history (e.g., pain of musculoskeletal origin, peripheral nerve damage, or disease of the central nervous system), and those with signs that resemble signs in one of the other categories but without obvious cause. Animals in all three categories can exhibit signs of psychologic or psychosocial dysfunction.
Some signs are likely to be common to chronic pain of any origin. They include decreased appetite, weight loss, reduced activity, sleep loss, irritability, and decreased mating and reproductive performance (Soma, 1987). Alterations in urinary and bowel activities and lack of grooming are signs often associated with chronic pain, and tearing and lacrimal accumulations around the eyes should be noted. Animals whose pain is chronic or that are moribund might exhibit reduced body temperature, a weak, shallow pulse, and depressed respiration—signs of a poor prognosis.
Chronic pain of the musculoskeletal system is fairly easy to recognize because of lameness or reluctance to move. Some chronic conditions can cause an animal to harm itself; e.g., licking can progress to rubbing, chewing, or scratching, and occasionally self-injury becomes so severe as to mask the cause. Even if a cause is identified and corrected, maladaptive behaviors might persist and require careful diagnosis and treatment (Chapter 5).
As with all surgical procedures, appropriate anesthesia should be used to render the animal insensitive to pain. However, the postsurgical period is most likely to be associated with pain; pain should be expected, and appropriate use of analgesics should be planned and described in the research protocol. Table 4-2 lists signs, degrees, and durations of pain associated with various classes of surgical procedures.
The Committee on Animal Research of the New York Academy of Sciences (NYAS, 1988) has developed guidelines for research that uses animals. Table 4-3 lists examples of experiments of various types and some ethical considerations relevant to them. The guidelines are of value to scientists who are designing experiments and to institutional animal care and use committee (IACUC) members who are trying to identify painful and distressful procedures.